What's the next big thing for "Silicon Valley," according to Microsoft CEO Steve Ballmer?

That depends on which Silicon Valley you are referring to.

According to Ballmer, there are really four different epicenters for technology—all of which get the "Silicon Valley" label.

Only two of them, according to Ballmer, are where the real action is right now—though the pendulum swings between them all the time.

So he told interviewer Reid Hoffman, the LinkedIn cofounder, at an event held by the Churchill Club in Silicon Valley Wednesday night.

Here's how Ballmer breaks down "Silicon Valley":

There's one "that actually builds silicon" and infrastructure products. Those are new chips, networking equipment, and storage devices.

"That still exists and it's really important," he said. "There's a huge amount of innovation going on at that level and it's based in Silicon Valley"—Silicon Valley as historically defined, which is roughly contiguous with Santa Clara County in the southern end of the San Francisco Bay Area.

He's paying a lot of attention to it for everything from partnerships, to sourcing products for Microsoft's own clouds. It's hot.

The second one is based in the city of San Francisco. It consists of all the consumer Internet companies.

There's a notion that "says if you are going to start a consumer Internet company," you have to do it in San Francisco these days, Ballmer says.

The third is "enterprise stuff," which he says is "a little dormant now," but is starting to come back into vogue with venture capitalists.

There's an investment pendulum that swings between consumer and enterprise: "Ho! We're only doing consumer! Ho! We're only doing enterprise!" Ballmer said in a caricature of investors' faddishness. It feels like the pendulum is swinging back to enterprise now, he added.

The fourth is the "big enterprise" companies. "Cisco is here, HP is here," he said. Microsoft also has a big campus in the traditional Silicon Valley world. This is "a fourth Silicon Valley not plugged into other three."

While many other places, including Seattle, have tried to replicate Silicon Valley by investing in universities and bringing in venture funds, there's one reason that they'll never beat it, Ballmer says.